1. Field of the Invention
The present invention relates to a long-life electron tube device, an electron tube cathode, and a manufacturing method for the electron tube device.
2. Description of Related Art
FIG. 1 shows an electron tube cathode used in an electron tube device of a cathode ray tube for a television or the like. The electron tube cathode is composed of a cylindrical sleeve 910, a base metal 920 that caps one end of the cylindrical sleeve 910, and a heater coil 940 provided inside the cylindrical sleeve 910. The base metal 920 has nickel as the main component and includes a reducing agent such as magnesium. Furthermore, an emitter layer 930 is emitter layer 930 is formed on the base metal 920. The main component of the emitter layer 930 is an alkaline earth metal oxide such as barium oxide.
After applying an electron-emissive material suspension whose main component is barium carbonate or the like as a precursor for barium oxide or the like on the base metal 920, an assembled cathode ray tube is subjected to a exhausting process in which the cathode is heated by the heater 940 to form an alkaline earth metal oxide from the suspension. The alkaline earth metal oxide is partially reduced so as to be activated to be semiconductive. Thus the emitter layer 930 is formed.
It is necessary for electron emission from an electron tube cathode to be stable over a long period of time in order to lengthen the life of the electron tube device. However, when the base metal 920 includes magnesium and the main component of the emitter layer 930 is barium oxide or the like, emission of electrons from the cathode is accompanied by formation of a composite oxide layer (hereinafter “intermediate layer”) composed of magnesium oxide or the like at the interface between the emitter layer 930 and the base metal 920. In addition to being highly resistant and therefore impeding the flow of current, this intermediate layer hinders diffusion of magnesium in the base metal 920 to the emitter layer 930, meaning that barium is not sufficiently produced in the emitter layer 930. This gives rise to a problem that emission characteristics that are stable over a long period of time cannot be obtained.
There are numerous studies in the prior art that attempt to solve the above-described problem by determining what kind of materials to use in the emitter layer 930. An example of such prior art is U.S. Pat. No. 5,146,131 which discloses a technique for adding 0.2 to 25 weight percent of europium oxide, ytterbium oxide, or lutetium oxide to the emitter layer.
However, development of an electron tube cathode with stable emission characteristics over an even longer period of time to lengthen the life of an electron tube apparatus is a perpetual problem.